1. Field of the Invention
The present invention relates to a recording apparatus such as ink jet printer and more specifically the present invention is directed to improvements in the ink supply system used in such recording apparatus.
2. Description of the Prior Art
Conventional recording apparatus generally includes a carriage having recording element(s) mounted thereon. To effect printing the carriage is driven on a recording paper in a scanning movement while supplying ink to the recording element(s) on the moving carriage. The method generally used for supplying ink to the recording element(s) is to use a flexible ink supply tube which is connected to the recording element(s) when printing is carried out. Through the flexible ink supply tube, ink is supplied to the recording element or elements from a stationary ink tank.
FIG. 1 shows an ink jet printer in which the above-mentioned ink supply method is employed.
In FIG. 1, reference numeral 1 denotes a recording paper, 2 is a paper feed roller by which the recording paper 1 is fed in the direction of arrow Y (secondary scanning direction), and 3 is a stepping pulse motor for driving the paper feed roller 2. 4 is a paper guide roller and 5 is a paper guide roller serving to form a printing plane. Designated by 6 is a multi-nozzle ink jet head disposed opposed to the recording paper 1 and running in parallel to the paper 1. 7 is a carriage on which the ink jet head is mounted. The carriage is reciprocally movable in the directions of arrow L and arrow R. The carriage 7 is driven running by a carriage driving stepping motor 8 through a timing belt 9. 10 is a slide shaft for guiding the carriage 7. The position of the carriage 7 is detected by carriage position sensors 11 and 12. 13 is a flexible circuit board for applying driving signals to the ink jet head 6. 14 is an ink supply tube for supplying ink to the ink jet head 6. The ink supply tube is a flexible tube one end of which is connected to an ink tank 15 and the other end to the ink jet head 6.
With the ink jet printer having the abovedescribed structure, printing is performed in the following manner:
For printing the carriage 7, together with the ink jet head 6 mounted thereon, is driven into reciprocal running by the stepping motor 8. In synchronism with the running of the head 6 on the recording paper 1, driving signals are applied to the individual nozzles of the head 6 through the flexible circuit board 13. In accordance with the applied signals, ink droplets are jetted from the nozzles toward the recording paper 1 to form an image on the paper. During the operation, ink is constantly being supplied to the head 6 from the ink tank 15 through the ink supply tube 14.
Since the ink supply from the tank 15 to the head 6 is continuously made through the flexible tube 14 during the running of the head 6, the flexible ink supply tube 14 also moves following the running of the carriage 7 during printing. Therefore, for this type of ink jet printer it is required to reserve a sufficiently large space enough to allow the flexible tube 14 to move without any trouble. However, since the ink supply tube 14 is flexible, its moving course is not fixed but variable case by case. Because of this, a large space must be reserved for the moving flexible tube 14. This large space occupied by the movement of the flexible ink supply tube 14 has prevented the construction of smaller printers. In addition, the unstable movement of the flexible ink supply tube has brought about various other problems in this type of printer. For example, the running load on the carriage 7 and the ink supply pressure to the head 6 are often varied due to the unstable movement of the flexible ink supply tube. These variations in running load and ink supply pressure in turn produce the problems of entrance of air into the head 6 from the individual nozzles and frequent change in flying course of the jetted ink droplets. All of these troubles will reduce the quality of images formed on the recording paper 1.
As a solution to the above problem it has already been proposed to use two ink tanks, the first one of which having a smaller ink capacity (it will hereinafter be referred to as sub-tank) and the second one having a larger capacity (referred to as main tank). Only the sub-tank is mounted on the moving carriage and the main tank is fixed in the main body of a printer. In printing, the sub-tank and the main tank are disconnected from each other. During only non-printing periods of time, they are connected into communication to supply ink. Such an ink supply system has been disclosed in U.S. Pat. No. 3,967,286 specification, and has been admitted to be very effective to eliminate the above-noted disadvantages of the conventional ink jet printer. However, the apparatus proposed by the aforementioned patent has the following drawbacks:
In the sub-tank there is used a liquid absorber such as cellular plastics or foam rubber to store ink in the sub-tank. Therefore, a relatively long time is required to supply ink from the main tank to the sub-tank. Furthermore, the amount of ink which can be contained in the sub-tank is limited considering the volume of the tank. If the liquid absorber is replaced with an ink pump to eliminate the disadvantage, then the problem of overflow of the supplied ink from the ink jet head when the supply is too great may occur.
The sub-tank is formed with an opening for ink feed. Because of the opening, the ink within the sub-tank is easily affected by the atmospheric pressure which causes a change of liquid level within the tank. With the change of the liquid level, the ink pressure in the individual nozzles also varies, which will result in change in amount and speed of the ink jetted from the nozzles. To prevent these troubles it is absolutely necessary to keep the liquid level within the sub-tank constant.